1. Field of the Invention
The present invention is directed to a magnetic storage unit with improved ability to read data.
2. Description of the Prior Art
In magnetic data storage devices such as disk and tape drives, one way to increase capacity is to increase the numbers of recorded tracks. Both disk and tape drives have seen a dramatic increase in the number of recorded tracks (often measured as the number of tracks recorded per inch of the media) during the last 20 years. Although capacity also has been increased by other ways, such as an increase in linear bit density and/or in the method of data encoding, the drive designers very often prefer an increase in track density, because the reduction in, for example, signal-to-noise ratio (S/N) is less severe than compared with the same S/N reduction experienced, for example, if the linear density, is increased.
However, increasing the track density may be difficult if the tolerances of the magnetic system and the drive itself are such that during a read back operation the read head cannot follow the recorded track. For example in a disk drive, the recorded track may not follow an exact circle as the disk spins around. Due to imperfections for example in the spindle motor or mechanical design of the spindle mounting, the disk may wobble. The recorded track will therefore not follow an exact circle, but a path which wobbles (very, often with a motion resembling a sinus curve) around the ideal circle track.
FIG. 1 shows a part of the ideal track path (drawn as a straight line) and a dotted line which shows the actual path of the recorded track during subsequent read operations.
The same problem of track position tolerance occurs for tape drives. For example, for longitudinal recorded tracks, wherein each track is recorded in the direction of the moving tape, the track will not follow the exact path during a later read operation as it did during the write operation. Again, it will typically follow a curve which varies around the originally, recorded track. Part of the reason for this behavior is the fact that the tape position typically is controlled by two or more tape guides, and there is often a small amount of play between the edges of the tape and the guides. Additionally, as the tape passes over the head, it has normally no guiding and may "float" slightly away from the nominal correct position.
Therefore, the track density, cannot be increased beyond certain limits which at least partly are determined by the media itself, and partly by the mechanical design and limitations of the drive, unless some methods for improvements are introduced to reduce this position variations. In disk drives, one very, common method has been to use an embedded servo system. Although the actual implementation may vary, the basic principle is that the disk contains prerecorded information (either on a separate disk platter or combined with the data area). This servo information is read by the disk drive as the disk is spinning and is used to correct for any small, rapid deviations from the correct position. Often a solenoid is used to quickly move the head to the correct position.
With this method, the read head when reading data from the disk will typically still follow a curve centering around the original recorded track, however, the deviation from the original curve is reduced and the read head is therefore able to read back the data.
Helical scan tape drives (like video drives) often utilize a different servo principle wherein deviations from the correct position are read by the drive, which in turn increases or decreases the tape speed slightly until the read head is exactly back on track.
Longitudinal recording tape drives such as 1/4" cartridge tape drives (often referred to as QIC drives) have mainly relied on mechanical improvements in the tape cartridge or the drive itself in order to increase the number of tracks. However, systems using tapes with special prerecorded servo information are now being introduced. The drive will have specially designed heads with several channels. Each channel has a certain distance from the other channels. During write or read operations, one of the channels will follow one of a set of prerecorded servo information tracks while the other channels will write or read data. The channel used to detect the servo track information will use this information to determine if the head is correctly following the servo track. If not, the information from the servo head will be used by the drive to reposition the head to the correct position. Many methods may be used for this repositioning operation, including the use of a small solenoid or a fast stepper motor.
FIG. 2 shows the front of a typical QIC drive head designed to follow such a servo operation. It has three channels, each with two read elements RA, RB, RC and one write element WA, WB, WC, where alternately one of the channels at a time is used to follow a servo track while either one or two of the other channels are used for recording or reading. The servo information is normally prerecorded on the tape at the time of manufacturing.
Therefore, as described above, the number of recorded tracks may be increased by the use of a servo system to correct for the small variations which will happen in any kind of system. However, to increase the number of tracks further, the complexity of the servo systems very often increases drastically. Also, in many applications, the cost and/or power requirements for a servo system are not acceptable.